The objective of this contract is to test approximately 100 chemicals for their ability to induce chromosome aberrations and sister chromatid exchanges in Chinese hamster ovary cells. All test chemicals will be provided to the contractor as coded samples. The contractor will use a standardized protocol developed by previous NTP contractors (Galloway et. al., 1985), which includes testing the chemicals both with and without exogenous metabolic activation. Some chemicals may be selected for in-depth analysis to determine mechanisms of action or possible protocol modifications.